CHAPTER TWENTY-ONE

RETURN TO CHICAGO

THE FERMIS ARRIVED BACK IN CHICAGO ON JANUARY 2, 1946. BY June they moved to a new house not far from the old one, at 5327 University Avenue. Like the one on Woodlawn Avenue, it was a grand, three-story turn-of-the-century brick edifice. This house served as the Fermi family home until 1956, when, after Enrico’s death, Laura moved to an apartment. Nella and Giulio both returned to the Lab School. Laura picked up where she left off in the summer of 1944, managing a busy social schedule, hosting numerous parties for faculty and students, many of which involved square dancing, often with Harold Agnew calling the moves. She started a book group, called The Paperbacks, which focused on a range of classics.

After the war, Enrico Fermi gradually became a more well-rounded, if not exactly worldly, person. He occasionally joined Laura’s reading group. Though the fact that he might not have read a book never prevented him from having strong views on it, the group did seem to broaden his philosophical and cultural interests. He commented to Segrè that he would occasionally persuade the group of his views by “using the old Italian method of shouting louder than his opponent.” In the summer of 1953, he was invited to speak at a seminar in Aspen for twenty young businessmen that was designed to expose them to a broad range of readings in history, culture, and philosophy. Fermi enjoyed the week and was amused to be considered a “philosopher” by the conference organizers. Fermi confided to Segrè that he had been thinking about the philosophical aspects of quantum theory, aspects that earlier held no interest for him. He did not, however, bother to commit any of those thoughts to paper. Amazingly, Laura even persuaded Enrico to attend a performance of the highly popular musical South Pacific, for Enrico a major concession to the musical arts.

IN THEIR TEENAGE YEARS, NELLA AND GIULIO EXPERIENCED THEIR father in different ways.

Nella admired her father and had the easier relationship with him of the siblings. At a conference at Cornell in 2003, she described her father trying to teach her algebra when she was eleven. He was not particularly successful, but laid the foundation, she felt, for a quicker understanding of the subject when it came time to study it at school. She also described his carpentry projects around the house. They were more functional than aesthetic. When Laura complained about the crudeness of one such project, Fermi observed that the work was hidden behind the couch, so no one would notice. As Laura stormed off, he turned to Nella and, in a highly revealing moment, advised his daughter, “Never make something more accurate than absolutely necessary.” The two of them would sometimes cook together when Laura was out of town. Not surprisingly, he could be quite literal in his interpretation of recipes. Nella remembers fondly the time when her father came home from the lab with a new substance for which he had been asked to think of some uses. It had the consistency of putty when pulled apart slowly but broke like glass when pulled apart quickly. He showed it to Nella and Giulio, but none of them could figure out what it might be good for. It was “Silly Putty,” which was a popular toy in the 1950s and 1960s. For a time they played with another new toy, the “dunking bird,” a plastic bird that, properly set up, would repeatedly dip its beak into a glass of water and then straighten up. They had good fun, even though, as Nella was quick to note, her father was never particularly demonstrative.

Giulio had a more troubled relationship with his father. A very bright boy with a tendency toward depression, he never felt comfortable in his father’s shadow. He rarely if ever spoke about his father in his later years, but he did occasionally confide in Robert Fuller, his lifelong friend from his undergraduate years. Giulio told Fuller of his frustration at not being able to build a working electric motor and his humiliation every time his father would step in to fix the problem at hand. He talked about his feelings of inadequacy when he compared himself to his father and of his father’s relative insensitivity to this. The problems Giulio faced came to a head when he was sixteen, about ready to attend college. He tried to commit suicide by slitting his wrists. Fuller notes that Giulio first realized just how much his father really cared for him in the ambulance ride to the hospital that terrible day. He recovered and, after a short and miserable spell at the University of Chicago, found some distance and peace at Oberlin College, where few of his fellow students knew of his relationship with one of the towering scientific figures of the twentieth century. By the time Giulio arrived at Oberlin, he had changed his name to “Judd,” a name he kept for the rest of his life. The effect was to create a distance from his famous father and his Italian heritage.

It wasn’t that Fermi was a particularly bad parent. He may have been inattentive, but no more so than other career-driven fathers in America during the immediate postwar period. Giulio had the misfortune of inheriting a delicate psyche, perhaps directly from his grandmother, Ida. His sensitive makeup was simply ill-suited to life with Enrico.

MORE GENERALLY, THE WAR CHANGED EVERYTHING. ENRICO FERMI was now a national figure, even a bit of a celebrity. The best-selling official report on the Manhattan Project was partly responsible for this. Journalists’ intense interest in the Manhattan Project made Oppenheimer and Groves superstars, but Fermi became famous as well, although to a lesser degree. He was inundated with invitations and requests for interviews for the remainder of his life. He participated in documentaries about the Manhattan Project, gamely re-creating for famed CBS broadcast journalist Edward R. Murrow the moment in December 1942 when he achieved a controlled chain reaction. He posed impishly in a famous photo session in front of a blackboard, which displays a formula for the fine structure constant that is clearly wrong. Because he knew as much about that constant as anyone alive, he almost certainly wrote it incorrectly to see whether anyone would notice. Along with four other University of Chicago alumni of the Manhattan Project, he was honored by the US Congress with the Medal of Merit. A plaque commemorating CP-1 was unveiled at a ceremony on the tenth anniversary of the event, presided over by University of Chicago president Hutchins.

FIGURE 21.1. Fermi at the blackboard during a publicity photo shoot. The equation for the fine structure constant alpha directly above his head is incorrect—most likely Fermi’s idea of a joke. Courtesy of Argonne National Laboratory.

Over time, however, as the invitations piled up, so did the graceful but firm letters declining them. Segrè suggests that Fermi might have begun to appreciate how little time he had left. Fermi may not have known that he would die so soon, but he certainly knew that, in general, physicists’ achievements tend to come early in life. He wanted to do as much as he could as fast as he could, while he still had the energy and mental acuity to do so.

IN THE SUMMER OF 1945, WALTER BARTKY, A UNIVERSITY OF Chicago astronomer, replaced Arthur Compton as dean of physical sciences. Bartky had the idea for new, interdisciplinary research institutes modeled on the Met Lab and Los Alamos. Endorsed by University of Chicago president Hutchins, Bartky invited Fermi to become head of an institute devoted to nuclear science. Though Fermi was enthusiastic about the new institute, he refused Bartky’s invitation to be the director, fearing that an administrative role would constrain his opportunities to pursue his own research. He suggested Allison for the role, and Allison accepted, seeing it as an opportunity to build a world-class scientific research institution, building on Fermi’s enormous reputation. Many of Fermi’s Manhattan Project colleagues eventually joined, and by 1950, the Institute for Nuclear Studies boasted some thirty-four senior scientists and a number of more junior staff, many of them either current or future Nobel laureates.

FIGURE 21.2. Fermi at the unveiling of a plaque at the University of Chicago commemorating the tenth anniversary of CP-1. University president Robert Maynard Hutchins is unveiling the plaque. From right to left, Fermi, Walter Zinn, Farrington Daniels, Robert Bacher, and William W. Waymack. Courtesy of Argonne National Laboratory.

Fermi may not have wanted to direct the new institute, but in a December 1945 letter to Bartky he detailed his thoughts on its research agenda. He wanted to use high-energy particle beams to explore the force holding together the atomic nucleus. Yukawa’s theory that the force was conveyed by a particle, the “mesotron,” was his starting point. Hitting a nucleus with enough energy might stimulate the creation of these particles, which would then be used to probe the nucleus.

Fermi urged Bartky to build a cyclotron capable of accelerating relatively heavy protons to a level of sixty million electron volts (MeV)—enough, Fermi believed, to create the conditions for mesotron production. Eventually, a souped-up cyclotron, capable of a then-astonishing 450 MeV, occupied most of Fermi’s experimental time when it came on line in 1951.

Fermi noted that during the five years it would take to build the cyclotron, cosmic-ray research could provide clues as to the behavior of mesotrons. This research would not require a cyclotron. Some of these rays were extremely high energy. Studying them could be useful in exploring the nuclear force in the absence of the cyclotron.

At the time he wrote this letter, Fermi was unaware that his Italian colleagues Conversi, Pancini, and Piccioni, working with makeshift equipment in a dingy Rome basement, had demonstrated conclusively that the mesotron was not the particle predicted by Yukawa, the manifestation of the field that holds the nucleus together. Their work had not yet reached the United States. The particle that did seem to match Yukawa’s particle was discovered in 1947, a bit more than a year after Fermi’s letter. It was originally called the pi-meson, a clumsy name that Fermi shortened to pion. The particle that his colleagues in the Roman basement studied eventually became known as the mu-meson, or muon, again a shortened name proposed by Fermi. In the context of what was known at the time, however, Fermi’s letter was a clarion call for research into the increasingly complex world of subatomic particles, a compelling agenda for the infant institute.

LIFE AT THE INSTITUTE WAS COLLEGIAL AND INFORMAL, A FAR CRY from the formality and hierarchy of the Met Lab under Compton. Office doors were typically open, and Fermi encouraged researchers to wander the halls and find out what their colleagues were doing. Fermi might have gone home for lunch, but just as often he would pick up colleagues and wander over to the faculty club, or he might have lunch with students at Hutchinson Commons. This was his favorite place to grab a hamburger and Coke during the Met Lab years, challenging his younger colleagues with thought-provoking questions like “How thick does the dirt on the windows of the Commons have to get before it begins to fall off the window?”

Wandering the halls was one of Fermi’s ways of finding out what was going on in the world of physics. He had stopped reading professional journals sometime before the war, relying instead almost exclusively on gossip with colleagues and institute visitors about interesting results and discoveries, challenging himself to figure out how the results were obtained. He also used these discussions to interest his colleagues in his current enthusiasms. One such enthusiasm was a concept called “spin-orbital coupling,” the notion that the spin of a particle or a set of particles within the nucleus could affect the way those particles orbited within the nucleus. In late 1949 and early 1950, he tried to get his young graduate student Richard Garwin interested in the concept, but Garwin showed no interest, so Fermi wandered down the hall to where Maria Mayer, his old friend from Columbia days, was working on her own project.

Mayer later vividly recalled the moment. She was working on why certain specific “magic” numbers of particles inside a nucleus resulted in particularly stable arrangements, using a model of the nucleus arranged in proton and neutron “shells,” similar to the electron shells surrounding the nucleus that determine the chemical properties of the various elements. She was getting nowhere. Fermi and Mayer chatted for a while, Fermi politely tolerating Mayer’s chain-smoking (he didn’t smoke and generally frowned upon people smoking in his office). They chatted about physics and the shell problem for several hours and then they were interrupted by a knock on the door. Fermi had received a phone call in his office. As he got up to leave, he casually asked, “What about spin-orbit coupling?” Mayer was thunderstruck: “Yes, Enrico, that’s the solution.” Fermi, always cautious, replied, “How can you know?” However, she immediately saw that spin-orbit coupling explained the problems with which she was struggling and within two weeks she successfully incorporated it into one of the first comprehensive theories of the nuclear shell model. Years later she would describe it with an analogy to a floor full of couples arranged in concentric circles dancing a waltz. Some circles are moving around the room in a clockwise fashion, others, counterclockwise; the couples are also each spinning themselves, some clockwise and some counterclockwise. In her analogy, each particle in the nucleus is like one of those couples; the overall effect of the relationship between their orbit and their spin influences the stability of the nucleus.

In 1963, her work was rewarded with a Nobel Prize, shared with Wigner and the German physicist J. Hans D. Jensen, both of whom also elaborated the shell model of the nucleus. She always credited Fermi’s generosity with helping her make the crucial breakthrough. Fermi had been generous indeed. Having seen the connection between the nuclear shell model and spin-orbit coupling, he could easily have done the work and published it himself. Instead, he offered the idea up and stood back, letting Mayer do the rest and take the credit. Mayer wanted to include him as a coauthor of the paper, but Fermi rejected that outright, explaining that because he was more famous, most people would assume he had done all the work, which was clearly not the case. It was a generosity that characterized his later years and reflected a more mature Fermi, comfortable in his stature as one of the world’s preeminent physicists.

The institute was exciting not only because of the staff or the easy collegiality that Sam Allison and Fermi set by example but also because it was a magnet for visitors from around the world. Many of the most celebrated physicists would visit and some would be invited to present at the institute’s biweekly theoretical physics seminar, organized by Maria Mayer’s husband, Joseph. These seminars covered a huge range of topics, all of which were cutting-edge science. Hans Alfven elaborated his theory of the intergalactic magnetic fields that Fermi believed were responsible for the high energy of cosmic rays. Feynman spoke about his work on liquid helium and its strange properties. Other visitors from the University of Illinois and the Institute for Advanced Studies at Princeton presented on topics of their choice. Most of the seminars, however, featured institute staff. Murray Gell-Mann presented on numerous occasions, as did Valentine Telegdi, Gregor Wentzel, and many others. In 1952 Fermi presented work on pion-nucleon scattering experiments he had just begun conducting at the newly operational cyclotron. These seminars ensured a continuing intellectual ferment within the institute community and kept everyone, including Fermi, abreast of developments in the world of physics. The seminars always started at four thirty in the afternoon. Staff members knew and visiting speakers were informed that they needed to finish by six o’clock. That is when Fermi would excuse himself and, irrespective of where the discussion stood, return home for the evening.

Gell-Mann recalls with some frustration Fermi’s tendency, if he disagreed with a speaker, to ask questions with the persistence of a terrier chewing on someone’s ankle. If Fermi raised his hand and said, “There is something here I do not understand,” the speaker was in for trouble and, if he had seen Fermi in action before, knew it. One senses that Fermi enjoyed disagreeing with Gell-Mann the same way he enjoyed disagreeing with his old friend Teller. It may have reflected an actual disagreement, but it was also a sign of respect.

IT WOULD TAKE FIVE YEARS FOR FERMI TO ACHIEVE HIS DREAM OF a major particle accelerator. Fermi was a consultant to the project and introduced a simple invention to remotely control the placement of the accelerator’s target without breaking the vacuum of the ring in which the protons traveled. Sam Allison oversaw the project, Herb Anderson took day-to-day responsibility for its development, and John Marshall assisted. The accelerator ended up growing from ninety-two inches to its eventual one-hundred-seventy-inch size, with a magnet weighing twenty-two hundred tons, and involved close, if sometimes fractious, collaboration among General Electric, Westinghouse, and the US Navy’s Office of Naval Research. It also involved a new technology, varying the radio frequency of the cyclotron voltage to maintain strict synchronization with the beam itself, in what is now called a synchrotron. The new technology was challenging to the electrical engineers constructing the machine, and Fermi regularly advised them on methods to overcome these challenges.

Because Fermi had no operational responsibility for the cyclotron’s construction, he faced a lengthy period during which he could not conduct high-energy particle experiments. What would Fermi do while he, and everyone else, waited for the new accelerator to be built? He wasn’t exactly the kind of person to bide his time. Though none of his postwar work had quite the lasting significance of the work he did before the war, he kept himself quite busy, in both experimental work and in theory. His pre-cyclotron research focus was on exploiting the neutron sources provided by CP-3 at Argonne. He also began a five-year study of cosmic rays and their origins and in the process developed a close professional and personal connection with one of the most unusual and important astrophysicists in the world.

AT ARGONNE, A NEW PILE, CP-3, HAD SUPERSEDED CP-2. IT WAS a fine source of neutrons that could be exquisitely controlled. For the next few years, Fermi undertook a series of important studies of neutron collisions using CP-3, collaborating mainly with Leona Libby but also with Herbert Anderson, Albert Wattenberg, and a number of younger graduate students. With Fermi’s old colleague, Walter Zinn, the new director of the lab, it was the old Fermi team working together again, as they had under the stands of Stagg Field in 1942, this time on projects selected solely on the basis of scientific curiosity.

Unburdened of his bodyguard-driver, Fermi enjoyed driving himself everywhere with characteristic, prewar gusto. Wattenberg was a frequent passenger on the forty-minute commute to Argonne. He recounts a harrowing moment when Fermi raced successfully to get across a railroad track before being blocked by an oncoming train. What Fermi did not realize was that there was a second track, obscured by the first train, and on that track was a second train going in the other direction. They missed the second train by a matter of a few feet. Pulling off to the side of the road so that the two of them could recover from what must have been a heart-stopping near miss, Fermi turned to his young colleague to reassure him. “This is why it is very important for you to be with me; my time may be up. But yours isn’t.”

At Argonne, Fermi and his colleagues bombarded some twenty-two elements with neutrons—slow, fast, and moderate in speed—to study how neutrons diffracted within solid material. They used a wide range of techniques invented by Fermi, including a “neutron mirror,” to measure the angular reflection of neutrons at various energies. Wattenberg recounts another characteristic Fermi moment. A group was looking for Fermi at Ekhart Hall, where the physics department was located, and someone told them that Fermi was in the machine shop in Ryerson Hall next door. They found him in discussion with one of the machinists, a fine and reliable tool and die maker with whom Fermi worked closely. Fermi was explaining to him how to use grit to produce the right kind of polish on a neutron mirror. The mystified machinist asked Fermi how to know if he had done the job properly. “I’ll hold the mirror up,” Fermi replied, “and if I can see my eyelashes it will be okay.” Not an especially rigorous standard for a high-precision instrument, particularly, coming from a Nobel Prize–winning experimental physicist, but it was characteristic of Fermi’s rough-and-ready approach.

These experiments, which continued through much of 1947, established an important base of information about how neutrons diffract through a wide variety of substances and were quite influential in setting the stage for research reactors to be used as test beds in materials science industry as well as in biological and medical research.

IN HIS 1945 LETTER TO WALTER BARTKY, FERMI MENTIONED THE importance of studying cosmic rays. The more these rays were studied, the more fascinating physicists found them. Some of these rays pounded the earth’s atmosphere with almost unimaginable energy, colliding with atoms in the earth’s atmosphere and creating a wide range of subatomic particles in the process. The universe, it turned out, was itself an enormous particle accelerator and, if a physicist was sufficiently diligent and armed with the right type of detectors, cosmic rays could be studied at very high energies, far higher than those achievable in man-made accelerators.

After the war, scientists debated the origins of these high-energy rays. Some, like Edward Teller, believed that they originated in the sun and that solar processes heretofore not understood were responsible for them. Others believed that they originated deeper in the universe, in interstellar space. There seemed to be no clear way to decide between these two propositions. These questions interested Fermi deeply, as did another question: How did they get their almost inconceivable energy? What could possibly accelerate these particles to such high speeds?

Fermi first began to think about these issues in 1947–1948, largely at the instigation of Teller. Fermi—perhaps, as he joked later, simply to find a way to disagree with his old friend—decided to argue that they come from deep space, well beyond the solar system. Thinking perhaps of how earth-bound accelerators push charged particles to ever higher energies, he constructed a theory to explain the high energies of cosmic rays. Fermi hypothesized that the presence of large-scale interstellar magnetic fields could explain the speeds with which cosmic-ray particles showered the earth. The Norwegian astrophysicist Hans Alfven visited Chicago in 1948 and gave a lecture arguing for the existence of these interstellar magnetic fields. Fermi liked the lecture enormously, because it added weight to his own theory of cosmic-ray acceleration, although Alfven always doubted the existence of fields strong enough to account for cosmic-ray energies.

Fermi soon found another foil for his musings on the origins of cosmic rays, with one of the most fascinating physicists of the twentieth century—Subrahmanyan Chandrasekhar. Born in India, “Chandra,” as Fermi came to call him, wrote his undergraduate thesis on the relationship between “Compton scattering”—the photon experiments that won Arthur Compton his Nobel Prize in 1927—and Fermi-Dirac statistics. Chandrasekhar won a doctoral fellowship to Cambridge University and did post-doc work under Max Born at Göttingen and Niels Bohr at Copenhagen. In short, he had as strong an academic pedigree as any major physicist working in Europe at the time.

While at Cambridge, Chandrasekhar was the first to predict that, for a star over a certain mass that runs out of nuclear fuel and goes cold, the gravitational force would be sufficient to overwhelm the degeneracy pressure implicit in Fermi-Dirac statistics, and the star would have no choice but to collapse upon itself to a point-like “singularity.” This mass came to be known as the Chandrasekhar limit, that is, the limit beyond which a white dwarf star would collapse into what John Wheeler later dubbed a “black hole.”^* However, Chandrasekhar’s work was initially ridiculed publicly by the powerful astronomer Sir Arthur Eddington, who made a point of ruining the young scholar’s career. Chandrasekhar had no choice but to leave England, and, in a move of striking prescience, the University of Chicago hired him in 1937. He won the Nobel Prize for his work in 1983 and remained in Chicago until his death in 1995.

Given the central importance of Fermi’s 1925 work on statistical mechanics to Chandrasekhar’s entire career, the only surprising aspect of the relationship between the two is that it seems to have started so late. Chandrasekhar arrived at the University of Chicago in 1937 but waited until many years later to send Fermi a rather formal letter inviting him to visit the observatory some one hundred miles northwest of Chicago on the shore of Lake Geneva, Wisconsin, in November 1947. Perhaps the delay reflected Fermi’s hectic schedule during the war or perhaps it reflected Chandrasekhar’s shyness. He might simply have been waiting respectfully until Fermi had settled into the new institute. Fermi responded positively and met with Chandrasekhar. The two of them hit it off well, the letters between them became increasingly less formal, and in the summer of 1949 Chandrasekhar invited both Enrico and Laura to spend time with him and his wife at their home near the observatory.

In the fall of 1953, the discussions between Fermi and Chandrasekhar became more regular. Chandrasekhar would spend two days a week on the Chicago campus, and the two would meet to discuss cosmic rays, galactic magnetic fields, and the like over lunch at the faculty club. In discussions with Chandrasekhar, Fermi extended the work he did in 1948, taking account of the spiral arms of the Milky Way galaxy and the magnetic fields created by them. The two men published several papers on the subject together during this period.

Fermi had no formal training as an astrophysicist, but Chandrasekhar likened him to a musician who, confronted with a new piece of sheet music, could perform it the first time through with the brilliance of an artist. For Chandrasekhar, the experience of working closely with Fermi was one of the highlights of his life. For his part, Fermi was willing to confide in his new friend, especially when it came to explaining how he thought about problems.

In retrospect, Fermi and Chandrasekhar were only partly correct about how cosmic rays develop such high energies. Astrophysicists still believe that magnetic fields are responsible for some of the high-energy particles that collide with Earth, but these are not necessarily free-floating interstellar fields. Rather they are associated with supernovae and with some highly unusual objects that were unknown in Fermi’s day, objects such as quasars, pulsars, and other highly dense, rotating objects with strong magnetic fields. Also, the universe is far larger than they knew then, with far more potential sources of radiation. Yet with all that was unknown at the time, Fermi’s method of studying the subject and coming to tentative insights demonstrates the power of his approach.

THOSE LIKE BRUNO ROSSI, WHO HAD BEEN STUDYING COSMIC RAYS, knew that particles like protons, pions, and muons were continuously colliding with the earth’s atmosphere, producing showers of other subatomic particles. The collision of pions and their subsequent decay into other particles was a subject of some interest to Fermi in the first part of 1947. Fortunately, he had a new graduate student who was not doing particularly well in his theoretical physics thesis topic and who was willing, indeed eager, to follow Fermi’s suggestion and conduct an experiment to study muon decay in the atmosphere. In the process, the student unwittingly demonstrated for the first time the existence of one of the fundamental forces of nature. His name was Jack Steinberger.

Steinberger, a German Jew who arrived in Chicago with his parents before the war, had not only been having trouble with his thesis but had trouble with the physics program from the beginning. He failed the basic exams required prior to beginning work on his doctoral thesis and was, thanks to the generosity of department chair Walter Zacharaisen, the only student ever to be given a second chance. Fermi clearly liked the young, strikingly handsome Steinberger. He asked Steinberger to be his teaching assistant for a course in elementary physics that Fermi taught in the fall of 1946. Fermi also agreed to be Steinberger’s thesis adviser after his student finally passed his basic exams.

Fermi could see that Steinberger was getting discouraged with his theory dissertation and suggested gently that perhaps an experimental project would be more to his liking. Fermi’s old friend Bruno Rossi and others had been studying the way cosmic muons, created by cosmic pion decay, themselves decayed into electrons and had discovered far fewer electrons than predicted. The rate was lower than expected by a factor of two. Sensing that it might be an interesting story, Fermi suggested that Steinberger study the spectrum of electron energies resulting from these decays.

It is a rare PhD dissertation that makes an important scientific contribution to the field, but largely because of Fermi’s sixth sense for important questions and also because of the unusual results of Steinberger’s experiment, this dissertation research was an exception to the rule.

Fermi lent Steinberger an assistant, and together they made some eighty Geiger counter detectors configured appropriately to capture electron tracks from cosmic muon decays. The first phase of the experiment was completed at sea level and later repeated in 1948 at the top of Mount Evans in Colorado, at an elevation of 14,271 feet, to enhance the statistical significance of the results. Steinberger discovered that the energy spectrum of electrons emitted from cosmic-ray muon decay was continuous and even at the highest energies was not sufficient to account for nearly half the energy that the muon’s decay should produce. Steinberger analyzed the data carefully and came to the conclusion that two neutral particles, “probably neutrinos,” accompany each electron emitted from the muon decay. These two particles carried off the missing energy.

Neither Steinberger nor Fermi realized the most important implication of this result. Steinberger says that he himself was not “clever enough.” It is difficult to see why Fermi missed it. Perhaps, as Steinberger suggests, “new ideas are not always easy to accept, sometimes even by the brightest and most open of people such as Fermi.” In any case, several others did, including three University of Chicago graduate students, Tsung-Dao Lee, Chen-Ning Yang, and Marshall Rosenbluth. They proposed that what Steinberger discovered was a more general extension of the “Fermi interaction” underlying beta decay. They suggested that there was a “universal” force—alongside gravity, electromagnetism, and the “strong” force holding the nucleus of an atom together—that produced changes in particles and that the neutrino always appeared in these processes as a way to account for the energy that was not imparted to the other particles created in these processes. This universal force came to be known as the “weak” force, and the creation and/or destruction of neutrinos came to be called the “weak” interaction, because the force responsible for this can only be felt at the closest of distances.

Steinberger recalls that Fermi was extraordinarily generous with his time, organizing the logistics and funding for Steinberger, but never interfered with the actual conduct of the experiment itself, allowing the young physicist to make his own mistakes along the way. Fermi explained to Steinberger, as he would to Maria Mayer at about the same time, that if Fermi added his own name to the paper reporting the results of the experiment, everyone would think that Fermi had done all the work, which would be bad for Steinberger.

CONFERENCES CONTINUED TO REVEAL AND DISCUSS IMPORTANT NEW developments in postwar physics, but the action shifted to the United States, where a series of major conferences organized under the auspices of the National Academy of Sciences effectively replicated the prewar Solvay conferences. The first was scheduled for early June 1947 at Shelter Island, situated between the north and south forks of Long Island, New York. Fermi received an invitation and was eager to attend. The agenda included discussion of some of the most interesting new developments in physics: Willis Lamb on a strange anomaly he had discovered while measuring the energy of the two possible quantum states of the hydrogen atom, an anomaly which would soon be known as the Lamb shift; Rabi on his precise experimental measurement of the magnetic moment of the electron, a value that Dirac’s theory of quantum electrodynamics could not compute; Robert Marshak on the Yukawa meson, whose experimental observation a few months later would vindicate Marshak’s speculations; and Feynman on an informal, preliminary presentation of his work in quantum electrodynamics and his use of a strange new analytical tool based on graphic diagrams of interactions. The stellar attendance list included among others Bethe, von Neumann, Oppenheimer, Rossi, Teller, Uhlenbeck, Weisskopf, and Wheeler. Fermi would have been in his element.

Fermi never made it to the conference. Passing through Baltimore, he noticed that his vision was blurred. Understandably alarmed, he decided to have a doctor examine him. The cause was a torn retina, which would take the better part of a year to heal completely. During this time, his friends would often notice him counting the ridges of his fingerprints or carefully holding a pencil in front of his eyes, trying to focus on the edge of the pencil to make sure his eye was returning to normal.

A second conference, at a resort in the Pocono Mountains of Pennsylvania, took place in late March 1948. By this time Fermi’s eyesight had recovered sufficiently for him to attend. In addition to the participants who were at Shelter Island, several other old Fermi friends attended, including Niels and Aage Bohr, Dirac, Wigner, and Wentzel. The main focus of the conference was quantum electrodynamics (QED), and two presentations on the subject made history.

For decades the niggling problem inherent in Dirac’s version of QED defied repeated attempts, including those by Fermi, to solve it. The completion of the magnetic moment of the election converged not on a finite value, as one would expect, but instead diverged to infinity. Between 1946 and 1948, three young physicists independently gave the problem another shot, and this time all of them succeeded. It was a great achievement, one of the greatest of twentieth-century physics, and put QED in an almost uniquely effective category of physical theory. Two of these physicists were invited to the Pocono conference—Feynman and a thirty-year-old Harvard professor named Julian Schwinger. (The third, a Japanese theorist named Sin-Itiro Tomonaga, independently developed a solution similar to Schwinger’s and would certainly have been invited to present at the conference if the organizers had known about his work.)

Schwinger and Feynman, though the same age, could not have been more different, as individuals or as physicists. Feynman was a gregarious and irrepressible showman; Schwinger was a quiet introvert, with no interest at all in entertaining his audience. As a physicist, Feynman’s presentation reflected an informal, intuitive approach that depended upon his diagrams, which eventually found universal acceptance. Schwinger was more of a formalist, relying on equation after equation, carefully sequenced, to get to his solution. His presentation was so long and tedious that in the end only two physicists lasted through the entire lecture: Fermi and Bethe. Fermi took it as a badge of honor that he stayed until the last.

Although he had great respect for Feynman throughout his life, Fermi clearly preferred the Schwinger approach, based squarely on the traditional quantum field theory Fermi knew so well. Feynman’s approach was anything but traditional, and the graphic tools Feynman used did not appeal to Fermi, at least not immediately. In 1951, Fermi and Franck jointly nominated Schwinger for a Nobel Prize. They cited Feynman’s and Tomonaga’s QED work, but they believed that “the greatest contribution was made by Schwinger.” Feynman never knew this. He continued to correspond with Fermi on a wide variety of physics problems and visited Fermi from time to time at Chicago. Feynman had enormous respect for Fermi, and part of his eagerness to bounce ideas off Fermi may have been an unconscious effort to gain the validation as a physicist that he sought but did not receive at the Pocono conference.

Why is it that Feynman, Schwinger, and Tomonaga succeeded in 1947 where Fermi (and others) failed in the early 1930s? Were they just better theorists? Given what we know about Fermi’s formal abilities in mathematical physics and his ability to crack profound problems in quantum field theory, such a conclusion seems, on the surface at least, too facile. Perhaps a better explanation is rooted in Peierls’s observation that Fermi was attracted to problems in which the mathematics was relatively simple. Peierls’s implication is that Fermi was an impatient theorist, that when a problem did not quickly succumb to his intellect he quickly lost interest in it. Another possibility: Fermi was an intensely practical physicist, not inclined to put more effort into a problem than he felt it was worth. It is entirely possible that he knew that the first approximation of the magnetic moment of the electron provided by Dirac’s theory was good enough for most practical purposes and that finding a way to calculate the value out to five or six decimal places was simply not worth his time and energy. As Fermi told his daughter Nella in another context, “never make something more accurate than absolutely necessary.” After the war it was not a problem to which he devoted any time. This is not to deny the historic achievement of these three, who shared the Nobel Prize for their work in 1965. Yet for Fermi, it may not have been that interesting a problem or perhaps he knew that by 1947 he didn’t have the youthful brilliance and intellectual stamina required to crack it and was happy to give others a chance. For whatever reason, he was clearly more concerned with other scientific matters.

ACADEMIC SUMMERS ONCE AGAIN BECAME PERIODS OF RELATIVE freedom, and Fermi took advantage of them as he had before the war. Most summers he spent six or eight weeks at Los Alamos, doing research on various projects, mostly classified, and spending time with friends, old and new. Soon after the war, the McMahon Act of 1946 placed the control of all atomic energy research under US civilian government monopoly, focused on a handful of national laboratories, including Los Alamos. Congress initially considered a more restrictive law that would have kept research under military control. Fermi, eager to present a united front with Oppenheimer, supported this along with Compton and Lawrence. However, other Manhattan Project scientists were outraged by continued military control of nuclear research and particularly unhappy with the four scientists who gave their support. Herb Anderson delivered a stinging critique of the group that included his old friend and mentor:

I must confess my confidence in our own leaders Oppenheimer, Lawrence, Compton, and Fermi, all members of the Scientific Panel,… who enjoined us to have faith in them and not influence this legislation, is shaken. I believe that these worthy men were duped—that they never had a chance to see this bill. Let us beware of any breach of our rights as men and citizens. The war is won, let us be free again!

Widespread opposition to the bill ultimately led its sponsors to withdraw it and replace it with a 1946 bill, sponsored by Connecticut senator Brian McMahon, putting the nuclear program under civilian control. It created the Atomic Energy Commission (AEC), whose first head was former director of the Tennessee Valley Authority David Lilienthal. It established a General Advisory Committee (GAC) of science and technology experts to advise the AEC. Oppenheimer was named the GAC’s chairman and Fermi was selected for a four-year term. Under the AEC, new labs at Los Alamos, Argonne, Oak Ridge, Hanford, and Berkeley continued working to refine the country’s atomic arsenal and pursued basic research on a variety of scientific fronts. The new director of Los Alamos, Norris Bradbury, was eager to bring Fermi back to the mesa, and Fermi was delighted to return, family in tow.

FIGURE 21.3. Summers at Los Alamos after the war were spent with old friends. Here is Fermi, third from left, on an afternoon jaunt with Hans Bethe, L. D. P. King’s son Nick (behind the wheel), and Edward Teller’s son Paul. Courtesy of Los Alamos National Laboratory.

Upgrading from wartime austerity, the Fermi family settled into one of the nicer homes on Bathtub Row and Enrico cycled to work, while Laura and the children socialized and pursued summer activities. He worked on a variety of classified projects, but two unclassified projects were of particular interest.

First, he became enamored with computers. Before electronic computers were readily available, he designed analog computers that could help him with his calculations. One such computer, built in Chicago with the help of graduate student Richard Garwin, was capable of solving Schrödinger equations. Another one, built at Los Alamos with the help of his old water boiler colleague L. D. P. King, used Monte Carlo methods to trace the path of a neutron through matter, simulating the paper and pen calculations Fermi did on neutron diffusion before the war. It was affectionately dubbed “Fermiac,” after the early electronic computer ENIAC.

By 1947–1948, von Neumann developed one of the first programmable electronic computers and it became the focus of attention at Los Alamos. The father of computational physics, a young Manhattan Project veteran named Nicholas Metropolis, used von Neumann’s machine to explore how physics equations could be programmed into a computer to simulate physical processes and predict the outcome of experiments. With von Neumann and Stanislaw Ulam, Metropolis invented the modern computerized “Monte Carlo” method of simulating stochastic physical processes. Fermi jumped at the chance to put his own equations into the computer, working with Metropolis on a wide variety of studies, including what would happen if a high-energy pion hit the nucleus of a relatively simple hydrogen atom. Several years later, these simulations allowed Fermi to compare his theoretical predictions with actual experimental results when the Chicago cyclotron went live. Later, in 1953, Fermi worked with Ulam and another computational physicist, John Pasta, to study the problem that absorbed him in his 1918 application to the Scuola Normale Superiore. They programmed the equations of a vibrating string into the Los Alamos computer and simulated its behavior. The resulting paper became an early and crucial contribution to chaos theory by showing that the string would return to a specified state at regular intervals, which was inconsistent with its expected ergodicity. In this respect it was a direct lineal ancestor of Fermi’s very early concerns about ergodic systems and their behavior.

A second project that captured Fermi’s imagination during summers at Los Alamos was “Taylor instability.” An important concept in hydrodynamics, Taylor instability refers to what happens when the surface between two fluids of different density—oil and water, for example—is perturbed in some way. The complex interaction between the two fluids on a surface is extremely difficult to model mathematically, but the phenomenon becomes extremely important in certain types of events, including nuclear explosions. Fermi and Ulam published several papers on this crucial aspect of hydrodynamics.

The periods at Los Alamos gave Fermi time to stretch his mind in the company of a group of extraordinary physicists and mathematicians. It also provided time for relaxation and exercise. Fermi continued hiking and fishing around the Los Alamos area, habits acquired during the war. He also enjoyed playing tennis with anyone who was willing to accept his challenge, as Ulam did frequently.^*

BY 1951, THE CYCLOTRON IN CHICAGO WAS UP AND RUNNING AT A then-impressive energy of 450 MeV, one of the most powerful cyclotrons in the world. Fermi had been preparing for this moment for some time. He summarized his preparations in lectures presented at Yale in the Silliman Lectures of April 1950, subsequently published in a short volume called Elementary Particles. In the lectures, he introduced a “statistical theory of pion production” providing a “plausible approximation” of high-energy collisions. When it turned out not to be an exact predictor, he resented the criticism, correctly pointing out that he had never intended it to be so. He also prepared a paper for presentation at the opening of the cyclotron in September 1951, an event attended by more than two hundred distinguished physicists from around the world. He had already conducted a few preliminary experiments and reported on these. The celebration gave him a chance to catch up informally with old friends.

He had thought carefully about what experiments he wanted to conduct and what might be interesting to explore. His main interest was an exploration of the “strong force,” the force that holds the nucleus of an atom together, by probing the nucleus with the pion, the particle Yukawa first suggested in 1935. In 1951, physicists believed the pion was responsible for the strong force and would interact in interesting ways with “nucleons”—that is, the protons and neutrons that dance together inside the nucleus. Importantly, the Chicago cyclotron was one of the only machines powerful enough to create pions with sufficient energy to probe the nucleus.

Fermi’s experiments, conducted with Anderson and a group of younger physicists, created a beam of pions by accelerating protons in the cyclotron to very high energies and then “smashing” them into a target. The protons stimulated the nuclei of the target to emit pions, and the pions were then used to probe other nuclei—in this case, hydrogen nuclei that consisted either of a simple proton or of a “deuteron,” a proton and a neutron. Pions hit these nuclei and bounced off—“scattering,” in physics terminology—and this scattering revealed interesting things about nucleons and their relationship to pions. The three different types of pions—positive, negative, and neutral—had slightly different masses, and keeping track of the way these different pions interacted with nucleons in the hydrogen nuclei told even more about nucleons.

In the process of studying these scattering patterns, Fermi realized that he produced a new particle, created when the proton in the nucleus was hit by a pion in a range of energies centering on 180 MeV. This was the first time protons were struck with such energy by a particle capable of exploring the strong force inside the nucleus. The particle, an extremely short-lived one, was at first known by its somewhat exotic quantum state and is now called the “delta plus plus” and fits quite neatly into the group theory framework of heavy particles (baryons) proposed by Murray Gell-Mann a decade later.

Physicists call this type of particle a “resonance.” Imagine that the nucleus is a radio station broadcasting at a number of different frequencies. The cyclotron is a radio receiver that can be tuned continuously to frequencies up and down the spectrum. When the listener starts tuning, only static might be audible, but then specific frequencies will come in and out of range, with clarity. In an analogous manner, the cyclotron produces pions that “tune in to” the nucleon at energies centering around 180 MeV, stimulating it to produce the new, somewhat exotic particle.

Analysis of these results suggested that protons were fairly complicated particles, which could be stimulated into a number of different states. Perhaps they were not “fundamental” in the sense of the muon or the electron. Perhaps they had an internal structure that could be further explored. It was an important insight that pointed the way to future developments in particle physics, developments that would entirely change the way these heavy particles were viewed. Fermi followed up these experiments with computer simulations at Los Alamos, aided by Metropolis, that were among the first particle simulations ever conducted.

Fermi wrote nine papers based on these experiments and computer studies. They were the last experimental papers he ever published. Herb Anderson writes that from mid-1953 onward, three Fermi graduate students—Jay Orear, Arthur Rosenfeld, and Horace Taft—ran cyclotron experiments that prevented Fermi from pursuing his own studies more thoroughly:

Through my illness [berylliosis], he lost a major supporter who was willing to help smooth the way and cater to his way of doing things. His new students, Rosenfeld, Orear and Taft asked his guidance and advice but wanted the work to be their own. So Fermi changed his role; he spent more and more time helping others by discussion and by frequently lending a hand in the experiment, but never again to the extent that would allow him to admit that the work was his own.

He goes on to suggest that Fermi’s decision to study the origins of cosmic rays with Chandrasekhar was a result of being sidelined by his graduate students.

Anderson was surely too critical of Fermi’s students. Fermi certainly did not need Anderson’s support to get time on the cyclotron. Fermi was the most prominent and powerful man at the institute and if he wanted to do an experiment on his own he certainly would have done so. He also had a long-standing fascination with the origins of cosmic rays dating back to 1947 and was happy to pursue conversations with Chandrasekhar without being forced to do so. What Anderson misses—perhaps through jealousy because Fermi was beginning to adopt a new, younger group of physicists to mentor—is that Fermi actually enjoyed working with his graduate students, encouraging their work, supervising their experiments on a big new instrument with such potential. Fermi justifiably had first crack at the cyclotron and by mid-1953, with two years of solid work under his belt, probably felt it was time to let his younger colleagues have a chance.

Anderson did have a jealous streak, as Richard Garwin found out. In late 1952, Garwin, who was as close to Fermi as anyone during this period, had a meeting with Anderson. The latter was rather direct: there was only room for one of them at the institute going forward. Garwin took that to mean Anderson and found a job at IBM’s Watson Lab in Yorktown Heights, New York. Anderson was rightly proud of his long-time collaboration with Fermi. Of all of Fermi’s collaborators in Rome and in the United States, Anderson probably worked with Fermi the longest. He was used to serving as a sort of gatekeeper, particularly for younger members of the team and resented it when physicists like Garwin or Rosenfeld developed their own special ties to the great man. Perhaps Anderson could be forgiven, though. He would eventually die of a disease contracted in a moment of heroism, when he dashed into a lab room ahead of Fermi and Zinn to put out a beryllium fire. In doing so, he spared both colleagues a similar fate.

FREEMAN DYSON, THE YOUNG THEORIST WHO DID SO MUCH TO reconcile the work of Feynman, Schwinger, and Tomonaga, provides a revealing coda to Fermi’s work on pion-proton scattering. Dyson was a junior professor at Cornell, responsible for supervising a small group of graduate students. They decided to tackle theoretical calculations of pion-proton interactions using the same technique successfully used to analyze quantum electrodynamics. The forces governing the pion-proton interaction are much stronger than those of electrodynamics, but Dyson and his students did not consider this a problem and got results that were fairly similar to those Fermi achieved using the Chicago cyclotron. This was the work of several years, and when completed in the spring of 1953, Dyson took a bus from Cornell to Chicago to show Fermi what they had done.

Dyson was eager to show Fermi his work. They chatted for a while about personal matters and then Fermi turned to Dyson’s results. Dyson recalled Fermi’s judgment on the work in 2004, some fifty years later: “There are two ways of doing calculations in theoretical physics,” Fermi explained. “One way, and this is the way I prefer, is to have a clear physical picture of the process that you are calculating. The other way is to have a precise and self consistent mathematical formalism. You have neither.”

Dyson was understandably stunned and asked Fermi to elaborate. Fermi explained that the mathematical technique Dyson used was inappropriate for the problem he was trying to solve. When Dyson objected that his results came very close to the numbers that Fermi himself had measured in his 1951–1952 experiments, Fermi pointed out that the arbitrary number of parameters undermined Dyson’s calculations. “I remember,” Fermi replied, “my friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” With that, Dyson made his way back to Cornell with the sad news that his work of several years had not passed Fermi’s test.

In retrospect Dyson was not bitter, but rather grateful that Fermi spared him from spending even more time on what was, really, a dead end:

Looking back after fifty years, we can clearly see that Fermi was right. The crucial discovery that made sense of the strong forces was the quark. Mesons and protons are little bags of quarks. Before Murray Gell-Mann discovered quarks, no theory of the strong forces could possibly have been adequate. Fermi knew nothing about quarks, and died before they were discovered. But somehow he knew that something essential was missing in the meson theories of the 1950s.… And so it was Fermi’s intuition, and not any discrepancy between theory and experiment, that saved me and my students from getting stuck in a blind alley.

THOUGH MUCH OF FERMI’S POSTWAR RESEARCH WAS PUBLIC, A significant portion of the summer work he pursued at Los Alamos throughout the period from 1946 through 1952 was classified, none more so than work on the hydrogen bomb. In this work, Fermi combined the roles of scientist and public policy adviser, the latter of which he found somewhat uncomfortable. Public policy was to become a major headache for Fermi and led to difficult, somewhat contradictory decisions. It also led to one of the most dramatic moments in his life, the defense of his old colleague J. Robert Oppenheimer against charges of disloyalty to the US government.

* A white dwarf star is the core remnant of a dead star in which the electrons and the nuclei are so compressed the atomic structure disintegrates, leaving the electrons in a degenerate state, compressed to the limit allowed by the exclusion principle. It is extremely dense, about two hundred thousand times the average density of the earth. National Aeronautics and Space Administration, “White Dwarf Stars,” Imagine the Universe!, revised December 2010, https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html.

* Ulam once won a match with Fermi, 6–4. Fermi refused to concede defeat. He pointed out that the difference between the two scores was less than the square root of the sum of the scores, 3.17. This is a shorthand method used by statisticians to determine whether a result is significant or within the limits of measurement error. Ulam found Fermi’s response at once ridiculous and adorable and continued to play tennis with his competitive friend. Adventures of a Mathematician, 164.